This disclosure relates generally to multi-chip electronic modules, more particularly those attached by land grid array (LGA) connectors.
Current and future high performance computer systems and server systems rely on both large scale packaging of multiple high density interconnect modules and boards that must be upgraded in service. Solder interconnection technology, traditionally used to interconnect smaller modules, is generally inapplicable to larger modules because of premature solder interconnect failures which violate reliability requirements. Moreover a module interconnected by solder interconnections is not easily replaced in the field. Mechanical attachment utilizing an array of brazed pins on the chip carrier and a mating socket on the board has been used for large multi-chip modules, and is readily field replaceable, but has a limitation on interconnect area density which precludes applicability for most current applications. A mechanically actuated interconnection technology, referred to as Land Grid Array (LGA) connectors has arisen, which provides removable and repluggable attachment capability of modules to boards and has sufficient reliability to meet product requirements. The LGA technology has consequently found widespread use in the electronics industry for large modules.
In most cases it is desirable that the removed module be disassembled such that one or more elements may be reused, thus gaining considerable cost savings. Included in module disassembly is the removal of the sealing cap from the chip carrier, or substrate.
Multi-chip modules are typically sealed with a cap to protect the interior chips, interconnections and other devices from atmospheric contamination and debris. The cap provides some degree of hermeticity, protection against debris and damage due to accidental handling, and may also be coupled to the back of the chips with a thermally conductive material to enhance heat transfer. In a typical package, a cooling apparatus, such as a heat sink or other cooling means is disposed on the cap proximate the module to remove heat created in the module.
In the past, there have been two principal technologies utilized to seal modules, adhesive sealing and compressive sealing. In the first, the cap is sealed to the periphery of the chip bearing surface of the chip carrier with an adhesive material, such as an elastomer film. In the second method, the cap is mechanically clamped at the periphery of the chip carrier by a polymer or metal sealing ring pressing on the top, or chip bearing, surface and a balancing support on the bottom surface.
To achieve full advantage of the upgrade and replacement capability inherent with the LGA technology, the LGA-attached module must be capable of being disassembled following an in-service field upgrade or module replacement. Disassembly of an adhesively sealed module is difficult, time-consuming and expensive; may leave a portion of the adhesive seal affixed to the module; and may cause damage to either or both of the cap or module.
Compressive seals are thus the preferred sealing technology for LGA modules. In the existing compressive seal technology, the carrier bottom surface is required as a clamping surface. However, with an LGA interconnect no structure can extend below the I/O pad surface without mechanically interfering with the printed circuit board (PCB). Moreover, utilization of the bottom surface reduces the area available for the input/output (I/O) pads. A clamping surface has been formulated by grinding a flange into the chip carrier or module, but this results in a significantly larger carrier by about 5–10 mm and incurs significant additional expense.
Thus a method and apparatus is desired to effect a reworkable compression seal which does not enlarge carrier size, avoids the need for grinding a flange into the carrier, and does not impact the bottom surface area available for I/Os nor the PCB physical design.